Most pressure relief valves feature a piston acting against a spring in response to the pressure in a tank or vessel in which they are located. When the pressure in the vessel reaches a critical value, the piston is designed to push against the spring and release a detent. This allows the valve to break its pressure seal with the tank causing the pressure therein to be relieved.
One of the drawbacks of these relief valves is that they usually cannot be used at very low tank pressures of about 2.5 psig or less. At these very low pressures, the valve cannot provide enough force at its sealing interface with the tank to maintain a bubble-tight or leak-proof seal. The leakage about the seal interface becomes especially acute as the critical pressure is approached.
At very low pressures, friction will also have more of an influencing effect upon the reproducibility and accuracy of the valve release.
For low pressure relief, it is, therefore, not uncommon to use valves having metallic or carbon rupture discs. These valves, however, cannot be reused after disc rupture, and are also generally of a larger diameter size.
This invention seeks to provide a pressure relief valve which can be used over a wider critical pressure range; is adjustable to release at different pressures; and can be used at very low critical pressures of about 2.5 psig or less.